Cellular solids are made up of interconnected networks of intersecting support members or plates that form the edges and faces of cells. Two classes of cellular metal solids have been developed: stochastic solid foams and periodic lattice block or truss materials. Such materials can be useful, for example, as building materials, as thermal insulation, protective packaging, electromagnetic shielding, protective shielding, battery components, to provide buoyancy and many other applications. Cellular solids can be made from a variety of materials, including metals, ceramics, polymers, composites, and semiconductors, and can be designed to possess useful combinations of thermophysical, chemical and mechanical properties that can be tailored by adjusting the relative density, cellular architecture or material type of the cellular solid.
There is growing interest in using cellular materials for applications where more than one function is required. The search for lighter and stronger load bearing designs along with improvements in stiffness and strength-to-weight ratio has been a major focus in the field of material science for many years. Modem research in this area has primarily focused upon relatively uncommon metals such as beryllium and titanium alloys, or fiber-reinforced composites materials. The composites typically consist of fibers (or whiskers) made of glass, carbon, polymer, metal, or ceramic (e.g., boron, silicon carbide or aluminum oxide) surrounded by a matrix (e.g., aluminum alloy, epoxy). While the exotic metal alloys and composites can provide efficient load support, and can be designed to simultaneously provide good mechanical impact energy absorption, they are expensive to develop and synthesize. Furthermore, metal and composite materials suited for load support and energy absorption are not often designed to simultaneously provide a high rate of fluid flow and heat transfer amongst other functionalities.
The porosity within a cellular solid makes it particularly attractive for multifunctional applications. Several classes of cellular solids have been developed, including gasars, consolidated powder products, vapor deposited materials, hollow sphere structures, stochastic foams and lattice block or truss materials. The most common are stochastic in nature, made by variants of foaming in the liquid, solid or semi-solid state. For example, stochastic metal foams can be made from directional foaming, which produces cellular architectures that are predominantly closed cell, often with wide distributions of cell size and many imperfections. Closed cell stochastic structures are useful, for example, for sound attenuation, fire retardation and impact energy absorption. However, they do not provide for fluid throughput (i.e., transport).
Fluids do however, flow through open cell stochastic foams because of the interconnected nature of the porosity. Materials of this type can be made using reticulated polymer foam templates. In one approach, the template is used as a pattern for an investment casting mold, which is then filled with a liquid (e.g., molten metal) and solidified. In another approach, a vapor or fine metal powder slurry is deposited directly onto the template. In the later, a subsequent heat treatment removes the organic compounds and densifies the structure. Open cell stochastic metal foams are useful, for example, for lightweight heat exchangers or as electrodes in nickel metal hydride batteries.
However, both open and closed cell stochastic foams have a number of disadvantages that preclude their use for many multifunctional applications. For example, current foaming methods do not provide for good control over the distribution of material at the cell level, leading to unit cells of non-uniform dimensions. Because of the non-uniformity, such structures do not normally allow fluids to easily pass through them. Furthermore, certain properties of both open and closed cell materials made by foaming techniques vary nonlinearly with their relative density. One disadvantage to this is that it complicates prediction of the physical properties based on the material and structural design. For example, the Young's moduli and compressive yield strengths of open cell stochastic metal foams vary with relative density according to the following power law relations:E/Es=(ρ/ρs)2 and  (1)σc/σys=0.3(ρ/ρs)3/2  (2)wherein E is the Young's modulus for the open cell stochastic metal foam, Es is the Young's modulus of its base material, σc is the compressive yield strength of the foam, σys is the yield strength of its base material, p is the foam relative density, and ρs is the density of the solid. The power law dependence on relative density indicates a rapid property loss with decreased density. It is a result of ligament bending. Stochastic metal foam structures do not generally provide a substantially linear dependence of the Young's moduli and compressive yield strength and exhibit low moduli and strength at low relative densities (e.g., ρ/ρs . at approximately 0.10)
Periodic metal truss structures have been made using polymer or wax truss patterns and investment casting. Recent work has used rapid prototyping and injection molding to create polymer or wax patterns with open cell lattice architectures followed by investment casting and heat treatment. These are known as lattice block or truss materials. Individual cells can be on the order of a few millimeters. By manipulating the truss architectures, the properties of the structure can be modified. For example, the Young's and shear moduli along with the tensile, compressive, and shear yield strengths of truss materials can vary with relative density in a substantially linear way when truss architecture is designed for tension or compression only with no bending (unlike open cell stochastic foam structures). This linear dependence is especially important at low relative density where properties far exceed those of stochastic foams.
By providing a substantially uniform and controllable distribution of material at the unit cell level, periodic truss structures can provide efficient load support in one or more directions, substantially isotropic and high convection heat transfer throughout the structure with low pumping requirements for fluid throughput in a direction, orthogonal to one or more load-bearing directions and excellent mechanical impact energy absorption amongst many other functionalities. However, the casting approaches used to manufacture miniature trusses are expensive and the resulting structures are subject to knockdown by casting factors (e.g., entrapped porosity, shrinkage residual stress, etc.). Furthermore, many materials of the conventional art are difficult to cast and do not favorably respond to post-processing (e.g. heat treatment).
Hence, there remains a long-felt need in the art for a multifunctional cellular solid that is characterized by the low production costs of stochastic foam structures, yet possesses the superior properties and multifunctional characteristics found in many periodic truss structures produced by investment casting. Accordingly, the present invention provides an inexpensive cellular material that can provide a unique and superior combination of properties and characteristics that overcome many of the disadvantages of the prior art. Specifically, the advantages of the present invention over the prior art include, inter alia, providing a cellular solid that possesses one or more, preferably all, of the following characteristics: (1) efficient support in one or more directions, (2) excellent mechanical impact energy absorption and vibration suppression potential (3) high heat transfer throughout, (4) low pumping requirements for fluid throughput, for example in a second direction orthogonal to one or more load-bearing directions, (5) substantially linear dependence of the Young's and shear moduli along with the tensile, compressive and shear yield strengths upon relative density and (6) a potentially inexpensive textile-based synthetic approach, (7) extendibility to any metal, ceramic, polymer, semiconductor or other material that can be woven, (8) excellent filtration potential, (9) a high surface area to volume ratio for enhanced activity as a catalyst or catalyst support, and (10) interconnected, open porosity for device storage, biological tissue in-growth or other functionalities requiring open space. Thus the teachings of the present invention overcome the limitations of the prior art by providing a multifunctional material that has a unique combination of advantages. The conventional art does not teach, provide, or suggest all of the important advantages of a multifunctional periodic cellular solid of the instant invention, as further discussed below.